1. Field of the Invention
The present invention relates to charging devices with magnetic particles.
2. Description of the Related Art
Many image-forming apparatuses based on electrophotography or electrostatic recording have conventionally been created. The schematic structure of such an apparatus and its operation are briefly described below with reference to FIG. 2.
When a copy-start signal is input to an image-forming apparatus shown in FIG. 2, a corona charging device 3 charges the surface of a photosensitive drum 1 to a predetermined potential. In addition, an integral unit 9, which includes a document-illuminating lamp, a short-focus lens array, and a CCD sensor, illuminates and scans an original document G placed on a document table 10. The scanning light is reflected off the document surface and is focused by the short-focus lens array to enter the CCD sensor, which has a light-receiving part, a transfer part, and an output part. The light signals entering the CCD sensor are converted into charge signals in the light-receiving part. The charge signals are then synchronized with clock pulses in the transfer part, are sequentially transferred to the output part, are converted into voltage signals in the output part, are amplified with reduced impedance, and are output to the outside as analog signals. The resultant analog signals are converted into digital signals by known image processing, and are transferred to a printer section. In the printer section, an LED exposure device 2 is turned on/off in response to the above image signals to emit light which forms an electrostatic latent image corresponding to an image of the original document G on the surface of the photosensitive drum 1.
A developing device 4 containing toner particles develops the electrostatic latent image into a toner image on the photosensitive drum 1. A transfer device 7 electrostatically transfers the toner image onto a transfer material which is then electrostatically separated and carried to a fusing device 6. The toner image is thermally fused onto the transfer material to output the image.
After the toner image is transferred, a cleaner 5 removes contaminants adhering to the surface of the photosensitive drum 1, including toner residue after the transfer. In addition, if necessary, the photosensitive drum 1 is exposed to light by a pre-exposure device 8 to eliminate the optical memory of the image exposure for repeated use in image formation.
Conventionally, charging devices for use in an image-forming process with an electrophotographic image-forming apparatus as described above are typically based on corona charging, as mentioned above. In recent years, however, contact charging has been intensively researched and developed, and has already been put into practical use. Contact charging has the advantages of yielding a small amount of ozone by discharge and low power consumption.
In contact charging, a charging member is brought into contact with a photosensitive member and is supplied with voltage to charge the photosensitive member. Examples of charging devices based on this method include charging rollers and magnetic brush charging devices. Among them, a magnetic brush charging device, in which a magnetic brush serves as a contact charging member, is used to achieve contact charging stability.
A magnetic brush charging device magnetically holds conductive magnetic particles directly on the surface of a magnet or on the surface of a sleeve including the magnet. The magnetic particles are brought into contact with the surface of a photosensitive member to charge the surface of the photosensitive member.
When a magnetic brush charging device is used to charge, for example, an organic photosensitive member having a surface layer in which fine conductive particles are dispersed (according to, for example, Japanese Patent Laid-Open No. 06-003921, which corresponds to U.S. Pat. No. 5,809,379) or an amorphous-silicon-based photosensitive member, the surface of the photosensitive member can be charged to a potential substantially equivalent to the DC component of a bias applied to a magnetic brush. Such a charging method is hereinafter referred to as magnetic brush injection charging. This method involves no discharging phenomenon as used in contact charging to provide ozone-free charging with low power consumption.
Unfortunately, however, a photosensitive member of an image-forming apparatus including a magnetic brush injection charging device has a shorter life than that of an image-forming apparatus including a corona charging device. The surface of the photosensitive member wears by friction against magnetic particles after repeated use. This problem is more serious when a sleeve and the photosensitive member are rotated in opposite directions to increase the possibility of contact between the magnetic particles and the photosensitive member for higher charging stability.
The life of the photosensitive member can be significantly increased by making a modification to the photosensitive member, such as a hard protective layer provided on the surface of the photosensitive member, or a modification to the charging device, such as the coating of the magnetic particles and the reduction of the pressure between the magnetic particles and the photosensitive member for lower friction against the magnetic particles.
The pressure between the magnetic particles and the photosensitive member can be reduced by, for example, extending the distance between the sleeve and the photosensitive member or reducing the amount of magnetic particles carried on the sleeve. Such approaches, however, also have adverse effects such as insufficient contact between the magnetic particles and the photosensitive member. This results in reduced charging stability and the adhesion of the magnetic particles from the sleeve to the photosensitive member.
The most serious effect among the adverse effects is the adhesion of the magnetic particles to the photosensitive member at the outermost ends of a magnetic particle carrying region of the sleeve. Magnetic particles adhering to the photosensitive member may undesirably intrude into a developing device to degrade image quality, cause transfer defects in a transfer device, and damage the photosensitive member at a cleaner.
The adhesion of the magnetic particles to the photosensitive member at the outermost ends of the magnetic particle carrying region of the sleeve results from an unstable magnetic particle carrying state. Such an unstable state occurs probably because the magnetic force of the magnet included in the sleeve is weak at the ends of the sleeve. In addition, the contact between the magnetic particles and the photosensitive member is poorer at the outermost ends of the sleeve than the center thereof. The photosensitive member is therefore insufficiently charged at the outermost ends of the sleeve to produce a potential difference between the sleeve and the photosensitive member. This potential difference can cause the magnetic particles to adhere to the photosensitive member. Some methods, including the insulation of the ends of the sleeve, have been proposed to prevent the problem of the potential difference between the sleeve and the photosensitive member.
If, additionally, the pressure between the magnetic particles and the photosensitive member is reduced to inhibit the wear of the photosensitive member, the magnetic particle carrying state becomes unstable at the ends of the magnetic particle carrying region of the sleeve. This is more likely to result in an insufficient effect of preventing the magnetic particles carried at the ends of the sleeve from adhering to the photosensitive member.